Drop dangling grid_mapping (and um_stash_source) from atmosphere CMOR output

[rhaegar325]

Summary

CMORised ACCESS-ESM1-5 atmosphere (Amon) files failed the CF check:

§5.6 grid mapping variable latitude_longitude must exist in this dataset

The fix drops the model-native grid_mapping attribute (and the related
um_stash_source provenance attribute) that leak from the raw UM variable into the
CMORised output. Affected all 9 Amon variables in the test set.

Root cause

1. The raw UM atmosphere files put grid_mapping = "latitude_longitude" on each source
   variable and include a latitude_longitude grid-mapping container variable
   (valid CF).
2. The atmosphere CMORiser's direct/formula path renames the source variable to the CMOR
   name (self.ds.rename({raw_var: cmor_name})), which keeps the source variable's
   attributes. update_attributes then merges the CMOR-table attributes on top with
   attrs.update(...), without clearing the inherited ones.
3. The latitude_longitude container variable is not carried into the CMORised output
   (only the target variable + coordinates/bounds are kept).

Net result: the output variable keeps grid_mapping = "latitude_longitude" pointing at a
variable that no longer exists → a dangling reference that fails the CF grid-mapping check.
The same mechanism leaks um_stash_source (a UM STASH code).

This was confirmed against the published reference: official CMIP6 Amon/tas has neither
grid_mapping nor a latitude_longitude variable — regular lat-lon grids carry no grid
mapping.

Change

src/access_moppy/atmosphere.py — in update_attributes, alongside the existing
attrs.pop("axis") / attrs.pop("positive") cleanup:

# Drop model-native attributes inherited from the source variable via the
# rename above that have no place in CMIP6 output:
#  - grid_mapping: a regular lat-lon CMIP6 grid carries none, and its
#    container variable is not carried into the output, so the attribute
#    is a dangling reference that fails the CF grid-mapping check.
#  - um_stash_source: a UM STASH provenance code, absent from the
#    published reference.
self.ds[self.cmor_name].attrs.pop("grid_mapping", None)
self.ds[self.cmor_name].attrs.pop("um_stash_source", None)

Tests

tests/unit/test_atmosphere.py — TestUpdateAttributesDecodedTime::test_model_native_attributes_dropped
sets grid_mapping and um_stash_source on the variable, runs update_attributes, and
asserts both are gone.

tests/unit/test_atmosphere.py: 56 passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.2%. Comparing base (09b4eb1) to head (d00c4ea).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #468   +/-   ##
=====================================
  Coverage   77.2%   77.2%           
=====================================
  Files         31      31           
  Lines       6030    6032    +2     
  Branches    1122    1122           
=====================================
+ Hits        4653    4655    +2     
  Misses      1126    1126           
  Partials     251     251           

Flag	Coverage Δ
unit	77.2% <100.0%> (+<0.1%)	⬆️

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